If you have full control over the protocol and both client and server implementation then I would consider a different method. It would be possible to continue using a end tag, but it can be difficult to do that correctly and reliably without fiddling with various selects and reading methods and hoping that piece of data won't appear in the file.

Alternativly I would do what some protocols do and use a message content length (in exact bytes) so that the reciver knows exactly how much binary data to read, after which normal operation can resume.

For this you could just add a line to the sender to like print OUT "Size: " . (-s $file) . "\n"; and the reciver can parse that line of data and use one or a series of read statements to recieve exactly that much data.